(1) Field of the Invention
The present invention relates to alleviating the transient loads on a towing cable provided between a surface vessel and an underwater vehicle, and deals more specifically with a means for accommodating a relatively short cable between the surface vessel and the underwater vehicle so as to avoid snap loads on the cable caused by the alternate lack of tension and tension loads created by the heaving motion of the surface vessel in a sea state with significant wave formation.
(2) Description of the Prior Art
Snap loads are a significant problem when towing heavy underwater bodies. Such tow systems are characterized by a tow cable of short scope with very little curvature since the cable shape is dominated by the body weight. In addition, the short cable leads to a very large incidence angle, that is the angle formed by the cable and the horizontal.
As the ship heaves downward due to wave motion, the cable becomes slack, and the body goes into free fall. When the ship heaves upward the cable suddenly becomes taut with the result that the towing vessel and the towed body or vehicle have different and perhaps opposite vertical velocity components. While this motion may tend to stretch the cable the inertia of both the ship and the towed body are brought to zero in a very short time as a result of a surge in cable tension (the snap load). Snap loads are particularly troublesome when the cable scope is relatively short as is generally the case for heavy underwater bodies being towed. The short cable has very little stretch because of its length.
Present systems for suppressing snap loads generally function by keeping the cable taut by compensating for this heaving motion of the towing vessel. Such systems have the drawback of causing cable fatigue at the point of contact between the tension reducing system and the cable itself.
The provision for buoys on floating cables is well known. However, these buoys are not designed to suppress snap loads in a tensioned or taut cable. They are generally used only to keep the cable afloat or to maintain it at a desired depth. Two patents that are exemplary of this general concept of floating cables are 3,698,348 and 4,313,392.
More typical for suppressing snap loads and towing cables are more complicated devices such as those shown in U.S. Pat. Nos. 4,964,491 and 5,140,927. The '491 patent discloses a system for alleviating snap load intensity by providing a spring hydraulic hooking device that deflects when a snap load is imposed on the cable. The '927 patent discloses a motion compensating and tension control system for the surface vessel, the cable being supported at the submersible vessel by a carriage with spring restrained motion that is intended to compensate for wave induced surface vessel motion.